The AIDS virus, HIV, utilizes the host cell factor P-TEFb to regulate its transcriptional and replication program. A small viral protein, Tat, and the 5'untranslated region (UTR) of the viral transcript are both involved in this process. Tat is an RNA binding protein that binds to the trans-activation response element, TAR, that is present in the 5'UTR. Tat can form a complex with TAR and P-TEFb. P-TEFb is a cyclin dependent kinase that controls the elongation phase of transcription, by regulating the transition into productive elongation. P-TEFb itself is regulated by the reversible association of a cellular RNA binding protein HEXIM1 or HEXIM2 and the small nuclear RNA, 7SK. This large form P-TEFb is relatively inactive compared to the free form of P-TEFb, but represents the major form of P-TEFb in many cell types. Therefore, most of the potential P-TEFb activity is held in this large complex. The original model for Tat transactivation emphasized the recruitment of P-TEFb to the nascent viral transcript;however, data recently obtained suggest that HIV utilizes the cellular P-TEFb control machinery more extensively than previously imagined. We plan to examine these broader regulatory mechanisms using a combination of biochemical and structural biological techniques. In this collaborative effort between a biochemistry lab and a structural biology lab, existing in vitro assays will be used to find conditions to generate relevant complexes containing proteins, or proteins and RNA that will form crystals. Finally the structures of these complexes will be solved using crystallographic techniques. The results from these studies will ultimately yield structural and functional information needed for the rational design of therapies targeting HIV, and will provide a basis for further understanding control of cellular transcription by P-TEFb. This project is aimed at understanding how HIV utilizes cellular P-TEFb regulatory machinery to control its own transcriptional program with the prospect of developing rational therapies for HIV. Complexes comprised of viral and cellular proteins and RNAs will be crystallized and structures determined.